The present invention relates to a rubber mixture for tire treads vulcanizable with sulfur, a method for its manufacture as well as a tire comprising a tire tread made of the vulcanized rubber mixture.
The service life of a tire tread depends on its composition. In cases in which a reduced rolling resistance together with improved grip on wet road surfaces is required, a rubber mixture is conventionally used for the tire tread that comprises a polymerized styrene butadiene rubber (SBR) polymerized in an organic solvent in combination with at least one further diene elastomer and finely divided precipitated silica as a reinforcing agent. The silica is used either alone or in combination with carbon black. Furthermore, the compositions for tire treads conventionally comprise mineral oil as a plasticizer. The vulcanization is carried out in the presence of sulfur and vulcanization control agents.
Due to the fact that the polar surface of the silica, in contrast to the surface of carbon black, is of reduced activity relative to rubber, it is necessary to treat the silica with relatively expensive silane coupling agents in order to ensure bonding of the silica surface to the rubber. This treatment is, in general, performed such that the silane coupling agent together with silica is added during the mixing process to the rubber. Conventionally, this mixing method requires exactly controlled mixing conditions due to the different reactivity of the different functional groups in most of the reinforcing agents with, in most cases, additional processing steps.
Furthermore, the grip on wet or moist street surfaces of tire tread rubber mixtures that contain silica in combination with a SBR rubber mixture is, in general, worse than conventional carbon black filled tire rubber mixtures. This is especially apparent in connection with vehicles that do not have anti-lock brakes.
The wear resistance of silica-filled SBR rubber systems is also reduced with respect to conventional rubber mixtures.
It is therefore an object of the present invention to provide a rubber mixture vulcanized with sulfur that is suitable for tire treads of pneumatic vehicle tires. Such tires containing the aforementioned tire treads should provide an improved sliding and rolling resistance without the aforementioned disadvantages. Especially, it is desired not to decrease wear resistance and steering precision. Furthermore, the tire manufacture, due to the use of inexpensive raw materials and reduced mixing expenditures, should be less expensive than with the aforementioned prior art rubber mixtures.